Lightning

Lightning that strikes the earth consists of one or more electrical discharges called strokes. The bright light that we see in a flash of lightning is called a return stroke. Return strokes travel at about the speed of light, which is 186,282 miles (299,792 kilometers) per second. They discharge about 100 million volts of electricity and heat the air in their paths to over 60,000 ºF. (33,000 ºC). Air heated by return strokes expands quickly, producing a wave of pressure called thunder.

Flashes of lightning vary in length. A flash between a cloud and the ground may be up to 9 miles (14 kilometers) long. A flash that travels through clouds side by side may be more than 90 miles (140 kilometers) long.

Through the centuries, lightning has been one of the greatest mysteries of nature--and it is still not understood completely. The ancient Greeks and Romans thought lightning was a weapon of the gods. In some African societies, people and places hit by lightning were considered to be cursed. As late as the 1700's, some people in Europe and America believed lightning could be kept away by ringing church bells.

Serious study of lightning began in the 1700's. In 1752, Benjamin Franklin showed that lightning is electricity. He tied a metal key to the end of a kite string and flew the kite in a thunderstorm. Cloud electricity raised the voltage of the kite string. The high voltage caused a spark to jump from the key to grounded objects, proving that the cloud was electrified. Franklin's experiment was dangerous, and some people who have flown kites in storms have been electrocuted by lightning. See Franklin, Benjamin.

Lightning is caused by the movement of positive and negative electrical charges toward one another. During a storm, a thundercloud's particles collide and become electrically charged. The positively charged particles rise to the top of the cloud and the negatively charged particles fall to the cloud's base. When negative charges from the cloud's base move downward and meet with rising positive charges from the earth, cloud-to-ground lightning occurs. Lightning between charges within a cloud is called intracloud lightning. That which occurs between charges of different clouds is known as cloud-to-cloud lightning. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagrams by Arthur Grebetz.

Electrically charged clouds. Scientists are uncertain exactly how cumulonimbus clouds become electrically charged. Most believe, however, that the charge results from the collision of a cloud's light, rising water droplets and tiny pieces of ice against hail and other heavier, falling particles. When these bits of cloud collide, the heavier particles gain a negative charge and the lighter particles acquire a positive charge. The negatively charged particles fall to the bottom of the cloud, and most of the positively charged particles rise to the top. Lightning is produced when separated positive and negative charges flow toward one another--or toward opposite charges on the earth--thereby forming an electric spark.

The most common type of lightning, called intracloud lightning, is created when charges within a cloud form an electrical spark. Charges that flow between a cloud and the air cause cloud-to-air lightning, and an electrical current between two clouds produces cloud-to-cloud lightning. Lightning that results from the flow of charges between a cloud and the earth may be either cloud-to-ground lightning, or ground-to-cloud lightning, depending on the direction in which the charges first began to flow. Most of the lightning that people see is cloud-to-ground lightning.

Strokes. The first stroke of a flash of cloud-to-ground lightning is started by a stepped leader, which usually carries negative charges from a cloud toward the ground. No one knows for certain how a stepped leader begins. However, many scientists believe it is triggered by a spark between areas of positive and negative charges near the base of a thundercloud.

A stepped leader moves downward in a series of steps, each of which is about 50 yards (46 meters) long and lasts about 1 millionth of a second. It pauses between steps for about 50 millionths of a second. As the stepped leader nears the ground, positively charged upward-moving leaders travel up from such objects as trees and buildings to meet the negative charges. Usually, the upward-moving leader from the tallest object in the area is the first to meet the stepped leader and complete a route between the cloud and the earth. Negative charges then rush down to the ground along the path provided by the upward-moving leader. The negative charge nearest the ground moves downward first, followed by negative charges from higher and higher altitudes. This process of the upward motion of current is the return stroke. A return stroke produces the light people notice in a flash of lightning, but the current travels so quickly that its upward motion cannot be perceived.

A flash of lightning may end after one return stroke. But in most cases, dart leaders, which are similar to stepped leaders, carry more negative charges from the cloud down the main path of the previous stroke. Each dart leader is followed by a return stroke. The leader-stroke process commonly occurs 3 or 4 times in one flash, but may occur more than 20 times. People can sometimes see the individual strokes of a flash. At such times, the lightning appears to flicker.

The major forms of lightning include forked lightning, streak lightning, ribbon lightning, and bead, or chain, lightning. Forked lightning refers to a flash in which multiple branches of a stroke are visible. Streak lightning is a flash that seems to illuminate a single jagged line. Ribbon lightning appears as parallel streaks of light. It is formed when wind separates the strokes of a flash. Bead, or chain, lightning is a flash that breaks up into a dotted line as it fades.

Some electrical flashes in the sky--such as heat lightning and sheet lightning--are not really separate forms of lightning, though they appear different in some ways. Heat lightning, often seen on summer nights, seems to occur without thunder. Actually, it is lightning that occurs too far away from an observer for its accompanying thunder to be heard. The people underneath what looks from a distance like heat lightning are experiencing a normal thunderstorm. Sheet lightning appears as an illumination of a portion of the sky. But it is lightning whose distinct flashes either are too far away to be seen or are hidden from view by clouds.

A form of lightning called ball lightning differs greatly from ordinary lightning. Ball lightning appears as a glowing, fiery ball that floats for several seconds before disappearing. It has reportedly been seen during thunderstorms, usually after ordinary lightning has occurred. It is described as a red, yellow, or orange ball that may be as large as a grapefruit. It has been reported floating along the ground and inside houses, barns, and airplanes. No one knows how or why ball lightning occurs, or what it consists of.

A glowing light called St. Elmo's fire may resemble ball lightning. St. Elmo's fire is caused by electrical discharges from a sharp object during a thunderstorm. It sometimes appears around airplanes, towers, masts of sailing ships, and treetops.

Take shelter in a house or large building. It is safe to stay in an enclosed car or truck, but not in a convertible. Do not touch any metal inside the vehicle.

Use the telephone only in emergencies.

If you are caught in an open area sit or crouch down.

Do not stand under or near an individual tall tree or any similarly isolated object in an open area.

Do not rise above the landscape by standing on a hilltop, on a beach, or in an open field.

Stay out of and away from water.

Get away from open metal vehicles, such as bicycles, golf carts, farm equipment, and motorcycles.

If you are stranded in a forest, take cover beneath low shrubs or a group of trees of similar height.

A person who has been struck by lightning should be treated with cardiopulmonary resuscitation, an artificial method of breathing and circulating blood.

Metal poles called lightning rods help protect buildings from lightning. They are attached to building tops and must be well grounded. They attract lightning and direct its electricity through a wire or cable safely to the ground. See Lightning Rod.

Contributor: Martin A. Uman, Ph.D., Prof. of Electrical Engineering, Univ. of Florida.

Kramer, Stephen. Lightning. Carolrhoda, 1992. Younger readers.

Pearce, Q. L. Lightning and Other Wonders of the Sky. Messner, 1989. Younger readers.

Uman, Martin A. All About Lightning. 1971. Reprint. Dover, 1986. Lightning. 1969. Reprint. 1984

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